Field of the Invention
The present invention relates in general to venting of air bag passive restraint cushions. In particular, the present invention relates to an improved frangible vent covering for such a cushion.
Description of the Related Art
Air bag passive restraint systems are increasingly common in contemporary vehicles. Such systems typically include an inflator for producing a quantity of inflation gas upon receipt of a signal indicating a collision. The inflator is connected to a cushion in the form of a flexible fabric bag. The inflation gas inflates the cushion to a predetermined pressure. During the collision the passenger is forced against the cushion, compressing the cushion.
During this compression the volume of the cushion is reduced. If the cushion were simply filled with the quantity of inflation gas, the pressure would rise in the cushion with the reduced volume until the pressure reached a point that the cushion was sufficiently hard that the cushion would injure the passenger. To prevent this, it has been known to provide vent holes in the cushion. The vent holes allow the inflation gas to exit the cushion in a controlled manner. This permits the pressure within the cushion to be relieved during the compression, providing a soft yet firm deceleration of the passenger, thus preventing injury.
While these vent holes are necessary for most inflator and cushion designs to operate properly during compression by the passenger, they produce undesirable consequences. Specifically, if the vents are fully open during the entire inflation process and during the time prior to compression by the passenger, the inflation gas will exit through the vent during these times. This means that a large quantity of inflation gas, sometimes up to 40%, is lost before the passenger compresses the cushion. This in turn requires that the inflator produce excess gas in order to have the required amount in the cushion at the start of compression. Building the inflator to produce excess gas of course increases the size, weight and cost of the inflator.
One solution which overcomes this problem is to provide a frangible covering over the vent, such that the vent is closed until the pressure rise during compression. When the pressure rises due to this compression, but before the pressure becomes dangerous, the frangible covering breaks and opens the vent. These arrangements thus permit only the actual necessary amount of inflation gas to be produced, yet still provide the required venting.
Examples of such arrangements are shown in U.S. Pat. No. 3,527,475 to Carey et al., U.S. Pat. No. 4,805,930 to Takada, and U.S. Pat. No. 5,219,179 to Eyerainer et al. While each of these arrangements is serviceable, certain drawbacks are still present. Specifically, Eyerainer et al. show the use of a perforation line in the cushion itself, forming a portion of a open polygon. Since the polygon is not closed, the entire section formed by the perforation will not be blown from the cushion, which is good. However, the open polygon necessarily results in two ends to the perforation line. There is always a worry that the perforation line will continue to tear from this line, opening a vent much larger than required and reducing the pressure to dangerous levels.
All three noted patents also show a standard vent opening covered by various patches. These closed polygon vent openings eliminate the worry of vent tears, but the patches provided are unduly complicated and expensive. Takada shows a patch having seams which tear the patch during distention of the patch caused by excess pressure. Eyerainer et al and Carey both show patches secured to the cushion about the vent, but which also include perforation lines. This is a simplification, but still includes drawbacks.
Carey shows an open polygon perforation line in the patch. Here there is again the worry that the tearing will not stop at the end of the perforations, permitting a section of the patch to be blown from the cushion, injuring the passenger. Eyerainer et al. also provide ends to the perforation lines, but do not form open polygons. As such, the possibility that the completed polygon will blow away is reduced. However, the X pattern of tear lines in Eyerainer does not full uncover the circular vent opening upon tearing, but instead forms an inscribed square. As such, the vent and opening must be designed together to provide the desired opening size in the patch, rather than in the vent. This makes modification of vent size more difficult.